When, back in the 1930s, physicist Erwin Schrödinger dreamed up his notorious thought experiment about a cat that is simultaneously dead and alive, he could hardly have imagined how it would enter the popular consciousness. Or how many terrible jokes it would spawn.

How’s this for a punchline, though: we still don’t know exactly what Schrödinger’s cat means. What you make of it will depend on where you stand on the fundamental question of where reality comes from.

From black holes to blockchains, from consciousness to the multiverse, we explain it all in this 13-part special

In the basic set-up you take a cat and stick it in a box rigged up with a radioactive atom, a hammer and a vial of poisonous gas. The atom decays, and this triggers the hammer to fall and break the vial, suffocating the cat.

Or not. Radioactive decays are random processes described by quantum theory, so we can’t say when one will happen. And quantum theory strongly suggests that before you observe or measure an object, it exists in a “superposition” of all its possible states. Before we open the box, the atom is both decayed and undecayed – and the cat both dead and alive.